1. Field of the Invention
The present invention relates to the preparation of amorphous or semicrystalline semi-aromatic (co)polyamides from an acidic monomer comprising at least one aromatic dicarboxylic acid having from 8 to 18 carbon atoms and from an amino comonomer predominantly comprising an alkylpentamethylenediamine.
2. Description of the Prior Art
Polyamides prepared from aliphatic diamines and from aromatic dicarboxylic acids have long been known to this art. Depending on their composition, these polyamides are: either semicrystalline polymers which have a high glass transition temperature T.sub.g, generally above 120.degree. C., and a melting temperature T.sub.m well above 300.degree. C., which is not compatible with facile melt-processing; or amorphous polymers which combine a T.sub.g which frequently exhibits medium values of 100.degree. C. to 120.degree. C. with a processing temperature which is not excessively high, on the order of 200.degree. to 290.degree. C. The semicrystalline polyamides are advantageously employed when it is intended, for example, to manufacture shaped articles which exhibit an excellent dimensional stability and an excellent retention of mechanical properties at high temperatures, as a result of the high value of the T.sub.g of the polymer employed. The amorphous polyamides are advantageously employed in fields which require, for example, an excellent transparency in the case of the fabricated shaped articles.
Semicrystalline polyamides of particular interest are those prepared from 2-methyl-1,5-pentamethylenediamine and terephthalic acid (cf. JP-A-69/019,551) because of the possibility of providing a T.sub.g on the order of 142.degree. C., while having a T.sub.m which remains below 290.degree. C. This permits the polymer to be processed according to the usual techniques employed for the conversion of polyamide 66. Amorphous polyamides of particular interest are those also prepared from 2-methylpentamethylenediamine, but reacted with a mixture of terephthalic acid and of isophthalic acid (15 to 30 mol % in the mixture of the diacids) (cf. FR-A-2,325,673). These latter polymers offer the possibility of providing a T.sub.g value which is high for an amorphous polymer and which, here again, can be as high as 142.degree. C.
A convenient operating procedure for preparing these semi-aromatic polyamides of particular interest entails the conventional polycondensation process used to prepare nylon 66, carried out in liquid phase or in the melt. According to this process, the polycondensation is carried out starting with compositions which either contain stoichiometric or essentially stoichiometric amounts of diamine and of diacid, or contain their salt, the operation being carried out in a closed system of the autoclave type, optionally in the presence of water and wherein the following stages are conducted in sequence:
(a) Stage 1: in which, with the autoclave closed, the temperature of the starting composition is progressively increased up to a value ranging from 200.degree. C. to 240.degree. C.; then, at a constant pressure equal to the autogenous steam pressure obtained, which, for example, ranges from 1.5 to 2.5 MPa when the starting composition contains water, removing the water present in the reaction mass by steady distillation by simultaneously progressively increasing the temperature of the mass to a value in the range of from 245. to 280.C;
(b) Stage 2: in which the pressure is progressively decreased from the value of the autogenous pressure to the value of the atmospheric pressure and, simultaneously, the temperature of the reaction mass is optionally increased to a value which is some ten to several tens of degrees centigrade higher than the temperature attained before decompression, while ensuring a steady distillation of water during this decompression period; and
(c) Stage 3: in which the polycondensation is completed by stirring the reaction mass for a certain period of time, the operation being carried out at atmospheric pressure and optionally (or) at a lower pressure with a mass temperature equal to or higher than the temperature attained at the end of Stage 2, until the point in time when the polyamide has attained the desired molecular and viscosity characteristics.
However, carrying out such a conventional polycondensation process is not free from disadvantages when the starting amino comonomer is an alkylpentamethylenediamine such as 2-methyl-1,5-pentamethylenediamine because of the development of interfering reactions involving this diamine. 2-Methyl-1,5-pentamethylenediamine is a compound which cyclizes readily; when involving the free diamine, this cyclization produces 3-methyl piperidine (a product designated hereinafter by the expression: "free cyclic amine") with a release of ammonia NH.sub.3 and, when it entails the diamine participating in the amidification reactions via only one of its functional groups, it serves as a chain-limiting mechanism, producing blocking groups of the formula: ##STR1## also with release of ammonia. The free cyclic amine formed is recovered during Stages 1 and 2 at the time when water is removed by distillation at constant pressure (Stage 1) and then during the decompression (Stage 2). Another interfering reaction includes the loss of the amino reactant (2-methylpentamethylenediamine) by entrainment, which takes place essentially completely during Stages 1 and 2 at the time of the removal of the water present by distillation at constant pressure (Stage 1) and then during the decompression (Stage 2). The result of these interfering reactions, therefore, presents two disadvantages:
(i) on the one hand, a high loss of total basicity, which is equal to at least 4.5%, involving, first, a departure from stoichiometry during the polycondensation between the primary amino groups and the carboxyl groups which react, consequently preventing the likelihood of readily increasing the molecular weight of the polyamide being formed and, secondly, an actual difficulty in reproducing the process on an industrial scale. The loss in total basicity described above is established in relation to the total amount of amino reactant introduced and is expressed by the equation: ##EQU1## the expression "basicity lost" corresponds to the sum: number of NH.sub.2 equivalents of amino reactant which is lost during distillation+number of NH equivalents of free cyclic amine+number of NH.sub.2 equivalents of ammonia. This lost basicity is measured directly, using potentiometric determination, on the distillates, i.e., on all of the water condensed during the distillation stages at constant autogenous pressure and during the decompression. It is possible to measure the number of NH equivalents of free cyclic amine alone, again using a potentiometric determination, by performing the operation on the distillates, but after they have been treated such as to differentiate between the free cyclic amine and other basicities (amino reactant and ammonia);
the expression "basicity introduced" corresponds to the number of NH.sub.2 equivalents of the amino comonomer introduced. The expression "number of" primary or secondary amino "equivalents" of a compound connotes the number of primary or secondary amino groups present in one mole of said compound; for example, 1 mole of amino reactant consisting of 2-methylpentamethylenediamine contains 2 primary amino NH.sub.2 equivalents, whereas one mole of cyclic amine consisting of 3-methylpiperidine contains one secondary amino NH equivalent; and
(ii) on the other hand, the existence in the polycondensation mixture of a high proportion of end groups of the cyclic amine type, which serve as a chain-limiter and can restrict the access to high molecular masses. It should be noted that this second disadvantage is less awkward than the first, relating to the loss in total basicity.
Overall, the above disadvantages associated with the use of an amino reactant such as an alkylpentamethylenediamine, capable of being readily entrained by distillation and cyclized by a reaction that produces products which are useless in the polycondensation, make it impossible to carry out the conventional process used to prepare nylon 66.